Heat-radiating materials comprising a silicone rubber or a silicone grease filled with a filler such as of alumina or boron nitride have been widely used as heat-radiating sheets or heat-radiating greases in a variety of kinds of electronic devices. The aluminum nitride excels in electrically insulating property, has a high heat conducting property, and is drawing attention as a filler for use in the heat-radiating materials.
In order to improve the heat conductivity of the heat-radiating material, it is important that a filler having a high heat conducting property is densely filled. When the aluminum nitride powder is used as a filler for the heat-radiating material, therefore, it is required that the particles forming the powder are spherical and have particle sizes of as wide as from about several tens of μm to several hundreds of μm. That is, in order for the filler to be densely filled in a medium such as a resin without impairing the formability (fluidity), it is most desired to use a powder that includes spherical particles of relatively large particle sizes as well as spherical particles of relatively small particle sizes so as to form a filled structure in which small spherical particles are distributed among large spherical particles.
As processes for producing the aluminum nitride, there have been known a process for reductively nitrogenating alumina, a direct nitrogenation process and a gas phase process.
The process for reductively nitrogenating alumina is a process in which a mixture of alumina and carbon is heated in nitrogen to reduce the alumina which is then nitrogenated to obtain the aluminum nitride.
The direct nitrogenation process is a process in which aluminum is reacted with nitrogen to obtain the aluminum nitride directly from the aluminum.
The gas phase process is a process in which an alkylaluminum is reacted with ammonia followed by heating to obtain the aluminum nitride.
According to the above processes for producing the aluminum nitride, however, it is difficult to obtain a powder of aluminum nitride that can be advantageously and densely filled in a medium such as a resin.
According to the reductive nitrogenation process and the gas phase process, for example, the powder of the obtained aluminum nitride comprises particles of nearly a spherical shape, the particle sizes, however, being mostly of the order of sub-microns.
According to the direct nitrogenation process, the aluminum nitride is obtained in a massive form and is pulverized and classified into particles of predetermined sizes. Therefore, the particle size can be controlled relatively easily. However, the particles assume cornered shapes which are far from the spherical shape.
Thus, there have been proposed aluminum nitride powders comprising particles having various shapes and sizes as well as processes for producing the powders, all of which having advantages and disadvantages. However, there has not yet been proposed any aluminum nitride powder having the above-mentioned particle properties and that can be densely filled in a medium such as a resin.
For example, a patent document 1 is disclosing an aluminum nitride powder comprising particles having a monotonous particle size of an average particle size of not less than 3 μm and having a round shape. However, the particles in the aluminum nitride powder do not have large particle sizes of not smaller than 10 μm.
Further, patent documents 2 and 3 are disclosing processes for producing an alumina nitride powder by reductively nitrogenating spherical alumina or hydrated alumina with a nitrogen gas or an ammonia gas in the presence of carbon. According to these processes, there can be obtained an aluminum nitride powder comprising particles of nearly a spherical shape and having relatively large particle sizes as well as an aluminum nitride powder having small particle sizes. According to the processes disclosed in these patent documents, however, the obtained spherical aluminum nitride powder tends to become hollow and, therefore, has a small particle strength and cannot stably maintain the particle size. That is, when added into the resin, the particles collapse into a fine powder and, therefore, deteriorate the formability (fluidity) of the resin. Besides, the particles are easily powdered deteriorating workability.
Further, a patent document 4 discloses the production of a spherical aluminum nitride powder by adding a forming assistant to an AlN powder produced by a predetermined method, wet-pulverizing the mixture thereof, granulating the pulverized mixture by using a spray drier, mixing a BN powder into the obtained granulated product (granules), and firing and sintering the mixture at a high temperature in a nitrogen atmosphere. According to this process, however, the firing is necessary for sintering the obtained particles in addition to the firing for nitrogenating the aluminum; i.e., the firing must be conducted twice at high temperatures. Besides, a step is necessary for pulverizing the aluminum nitride powder that is once produced. Therefore, the cost of production becomes too high, and it is difficult to carry out the process on an industrial scale.
Further, the aluminum nitride powder according to this process is obtained through the sintering. Namely, the particles are bonded together and are subject to be deformed due to the sintering. Further, though the strength against the crushing increases due to the growth of the aluminum nitride crystal particles, the particles tend to be easily dented or protruded to a large extent. Therefore, the obtained aluminum nitride powder has a small specific surface area, adheres less closely to the resin into which it is introduced, and the obtained heat-radiating material possesses insufficient strength.
A patent document 5 discloses a process for obtaining a crystalline aluminum nitride powder by maturing (heat-treating) an aluminum nitride powder of amorphous particles in a flux of a compound of an alkaline earth element or a rare earth element so as to assume a spherical shape, and dissolving the flux to obtain the crystalline aluminum nitride powder that is isolated. This process makes it possible to obtain the aluminum nitride powder of a shape and particle size suited for being highly densely filled. However, the aluminum nitride powder that is once produced must be, further, subjected to a special treatment casting a problem from the standpoint of production cost. Besides, the aluminum nitride powder obtained by this process contains impurities in large amounts due to the use of a flux agent.